The world's grief and compassion returned to the Indian Ocean's tsunami-battered shores Monday, marking a year since giant waves crashed ashore in a dozen countries, laying waste to communities and sweeping away at least 216,000 lives.

Beside gentle seas that belied the fury unleashed last Dec. 26, survivors, friends and relatives commemorated the lost lives and destruction from one of the worst natural disasters in memory. World leaders sent condolences.

In Indonesia's Aceh province on Sumatra island, the closest land to the magnitude 9 quake that spawned the waves, the president sounded a tsunami warning siren to start a minute's silence at 8:16 a.m. (0116 GMT) — the moment the first wave struck.

"It was under the same blue sky, exactly one year ago that mother earth unleashed her most destructive power upon us," President Susilo Bambang Yudhoyono said. "The assault began with a massive earthquake but ... that was only a prelude to the horrific catastrophe to come."

Similar silences were observed in Thailand and Sri Lanka, where President Mahinda Rajapakse joined ceremonies near the site where the raging waves swept a passenger train from its tracks, killing nearly 2,000 people.

In India, thousands took part in interfaith prayers at an 18th century church, then marched to a burial ground for tsunami victims.

Muslim, Christian, Buddhist and Hindu prayers services were being held across the tsunami zone; flags were lowered to half-staff, and bells rung.

Some preferred more personal reflection.

On Thailand's Patong beach, one man sat weeping in the sand before a gently lapping sea, a bouquet of white roses in front of him. He declined to talk to a reporter.

He was among hundreds of Westerners, including survivors and relatives of those killed, who traveled to sites along Thailand's world famous beachfront where their loved ones died or disappeared.

"Somehow it's good to see this place," said Ulrika Landgren, 37, of Malmo, Sweden, tears leaking from behind her sunglasses as she visited the beach where nine family friends died.

Last year's Dec. 27 quake — the most powerful in 40 years — ruptured the sea floor off Sumatra, displacing billions of tons of water and sending 30-yard-high walls of water roaring across the Indian Ocean at jetliner speeds.

Their impact was staggering.

The tsunami swept away entire villages in Aceh and Sri Lanka, swamped five-star resorts in Thailand and surged into coastal communities from India to east Africa.

On that day, Muhammad Yani clung to the second floor of an Aceh mosque, watching waters full of people and rubbish roil past him.

"I was not afraid at the time," said Yani, 35, who later learned that his parents and a younger brother were killed. "I was more aware than ever that my soul belonged to Allah."

Indonesia for the first time tested its tsunami warning system in Sumatra on Monday — a chilling reminder that the island sits on one of the world's most unstable geological fault lines and is still vulnerable. Scores of powerful aftershocks have rumbled through the region all year.